1. Field of the Invention
The present invention relates to high voltage drivers, and more particularly to converting a DC signal to high voltage AC signals and multiplexing the high voltage AC signals to multiple loads, such as electroluminescent lamps and the like.
2. Description of the Related Art
Electroluminescent lamps (EL) are essentially capacitors with one transparent electrode and a special phosphor material in the dielectric. When an appropriate AC voltage level is applied across the EL terminals, the phosphor glows. The brightness of a typical EL lamp is a function of the square of the voltage times the frequency, which is an approximation that holds over the useful range of most lamps. Common applications include back lights for digital displays, such as found on watches, pagers, cell phones, GPS devices, etc. Watches were a first application of EL lamps, which typically required about 80 to 100 Volts (V) peak-to-peak and a frequency of about 100 to 200 Hertz (Hz). Pagers required a slightly brighter display and higher voltage and frequency levels (e.g., 100 to 200 V and 300 Hz). Cell phones pushed the peak voltage to about 200 V. A more recent application of EL lamps include warning or indicator lamps for use in automobiles. Auto warning lamps require a light bright enough to be clearly seen in daylight, which has pushed the voltage upwards to a peak-to-peak voltage level of about 700 V.
The required AC voltage for automobile use has a relatively high magnitude on the order of several hundred volts (V) (e.g., 350 V). The required AC voltage, however, is not typically present in most systems and must be generated from a low voltage DC source, such as, for example, the 12 V battery. One proposed approach is to employ a crude boost switching converter and an H-bridge circuit to develop the requisite high AC voltage. The converter converts the low DC voltage to a high DC voltage, and the H-bridge circuit converts the high DC voltage into the high AC voltage consisting of two out-of-phase AC voltage signals. It would then be necessary to multiplex the two high voltage, out-of-phase AC signals to each pair of terminals of the lamps to be lighted. The multiplexing function would require 4-quadrant switches on both terminals of each lamp.
There are many problems with the proposed approach. It would generally require a 2-chip approach including a first driver chip designed to convert the relatively low DC voltage into the two out-of-phase AC voltage signals and a second chip to multiplex the AC voltages to selected ones of multiple lamps. Although driver chips for EL lamps exist, a driver capable of generating the very high voltage levels necessary to produce the high brightness required for automobile warning lights does not currently exist and would require substantial cost to design and manufacture. The second chip would also have to be designed and manufactured and would need to include 4-quadrant switches for each lamp. Each 4-quadrant switch would have to operate in a bidirectional manner to both source and sink current when activated and to block high voltage levels when turned off (up to approximately 350 V in both polarities). It is desired to provide a simpler and more cost-effective approach.